In recent years, to achieve a high fuel efficiency and a high power generation efficiency, there has been proposed a power generation plant which employs a huge reciprocating gas engine as a driving source of a generator. As an ignition method of an air-fuel mixture in such a gas engine for power generation, there is known a spark ignition method which ignites the air-fuel mixture by a spark generated between electrodes of an ignition plug. In order to optimize the efficiency of the gas engine and further promote energy saving in the power generation plant, it is considered that it is necessary to activate the gas engine in a state in which an ignition timing is advanced up to MBT (minimum advance for best torque). However, as the ignition timing is advanced, a temperature and a pressure in an interior of a combustion chamber increase. This may lead to a situation in which a knocking is more likely to occur and NOx emissions increase. If the knocking occurs frequently, the engine may be damaged, and NOx may pollute air.
Patent Literature 1 discloses a knocking suppressing system for a gasoline engine in a vehicle. According to this control system, when a running state of the engine is within a high load range, water is injected toward a cylinder. In this control, it is expected that the cylinder is cooled by latent heat of evaporation of the water, and thus occurrence of a knocking is suppressed.